The present invention is a tunable laser oscillator with a broadband emitting and thereby wavelength tunable laser medium. An optical resonator is provided with a mechanism for setting the wavelength of the laser radiation. A device immediately following the resonator for spectral filtering produces a spectrally purer emitted laser radiation. An aperture is mounted in the path of the radiation following the device. The aperture can be designed in several different ways.
The basic set-up of tunable lasers is generally known, for example, from the German laid-open publication DE-OS 27 15 844. They consist of a laser medium capable of broadband laser emission, for example a cell which is filled with a solution of organic dye and thus functions as the laser medium. This laser medium is excited by means of a pumping light source--for example, optical focusing elements. The laser medium in the cell is located within a tunable optical resonator, i.e., the resonator has a mechanism for setting the wavelength of the laser radiation. The optical resonator itself has an output coupling mirror and a back reflector mirror which are aligned to each other. The mechanism for setting the wavelength i.e. (the mechanism for tuning) consists of a rotating diffraction grating on a pivot. The diffraction grating also partially takes over the function of the back reflector mirror.
The spectral distribution of the laser emission i.e. the radiation which is coupled out of the resonator with the aid of the output coupling mirror, is broadband and thereby extends over a wavelength range. The laser radiation contains light from the wavelength selected with the wavelength tuning mechanism, but in addition it also contains broadband spontaneous radiation at a wavelength corresponding to the maximum amplification of the laser medium.
This broadband spontaneous radiation is known as ASE (Amplified Spontaneous Emission). The intensity of this broadband spontaneous radiation increases when, with the aid of the tuning mechanism, a wavelength is chosen which has at the end of the tuning curve, for example, one that is not amplified very well. However, the intensity of this broadband spontaneous radiation can also increase if the setting mechanism is not very effective, i.e., when very little light returns in the laser medium for amplification. In both cases, the energy not converted into the laser radiation of the tuned wavelengths will, to a large extent, be converted into broadband spontaneous radiation, ASE, since, due to the high amplification, only a small amount of light suffices to create a strong broadband spontaneous radiation.
If the laser beam is further amplified, then due to the reasons indicated, the relation of the intensity of the radiation of the tuned wavelength to the broadband spontaneous radiation, will deteriorate even further, if the tuned wavelength is in the low amplification range. The broadband spontaneous radiation, ASE, which arises in an interference nuisance factor since, generally speaking, the laser should be a light source where the laser radiation emitted is spectrally pure.
It is already known how to position a device for spectral filtering and for producing a spectrally purer emitted laser radiation behind the resonator of a tunable laser oscillator. (R. Wallenstein, T. H. Hansch in Optics Communications 14,353 (1975)). By this means, only the laser light of the tuned wavelength is further processed. The disadvantage of the known device is that the mechanism for setting the wavelength within the resonator and the device for spectral filtering and for producing spectrally purer emitted laser radiation following the resonator have to be tuned to each other synchronously. This means that not only the operation of this known tunable laser (from which the invention proceeds) is complicated, but also that its construction is quite expensive.
The object of the present invention is to provide an improvement of a tunable laser oscillator, as previously described, so that both the cost of construction and the method of operation would be simplified.